1. Field of the Invention
The invention relates to a process and apparatus for boronising pieces made of metal or cermet and to surface-boronised pieces.
2. Description of the Prior Art
A process is known for the treatment of pieces made of a material from the group comprising alloys of metals of the iron family (Fe, Ni and Co) and cermets, in which process the pieces are heated to an operating temperature of the order of 850.degree. to 1,150.degree. C., in the presence of a solid boronising agent and the boronising is activated by simultaneously subjecting the pieces to the contact action of a stream of a gaseous fluorine-containing agent, under defined operating conditions regarding pressure and temperature.
A process of the type referred to above for the boronising of steels is known from French Patent No. 2,018,609 and the equivalent U.S. Pat. No. 3,673,005, in which process the activator is a fluoroborate which is mixed with the boronising agent, in the presence of borax and to which a diluent consisting of alumina is optionally added. The whole reaction takes place in the solid phase and makes it possible to obtain a coating in which two phases are observed, one phase being FeB and the other being Fe.sub.2 B. However, the different crystal structures of these two phases create tensile stresses, on cooling, which detract from the high strength of the coating, all the more so because the FeB phase is more fragile and this leads to risks that the coating will flake off. Furthermore, it is observed that the pieces boronised by this known process retain traces of adhered powder because of the appearance of a molten phase, whereupon they must be subjected to an additional treatment in order to remove the more or less fritted powder which adheres to their surface to a greater or lesser extent.
Moreover, since the activating agent, which is consumed, is present in the treatment bed, it must be replenished, for example, a quarter at a time, with fresh powder after each treatment operation.
Furthermore, it is known that the same process can be applied, with the same advantages and disadvantages, to cermets, in particular to tungsten carbide or titanium carbide, enclosed in a cobalt matrix. Reference can be made, for example, to the article by G. L. Zhunkovskii et al, Boronising of cobalt and some cobalt-base alloys--Soviet Powder Metallurgy, 11 (1972) pp. 888-90 and to the article by O. Knotek, et al, Surface layers on cobalt base alloys by boron diffusion--Thin Solid Films, 45 (1977) pp. 331-9.